In the start-up of a new or rebuilt furnace for melting glass in a glass fiberizing process it is necessary to run the furnace for a period of time until the melting process has stabilized and impurities from the construction, such as particles of refractory material, have been flushed from the system. The molten glass in the system during this period is drained through drain bushings, which are electrically heated to very high temperatures to maintain the glass at the proper viscosity and which typically have two or three large drain holes. This is in contrast to the great many very small apertures contained in the fiberizing bushings used in the production of glass filaments.
Fiberizing bushings are normally formed of precious metal, such as platinum/rhodium alloys, which are quite expensive due to the cost of the precious metal and the cost of fabrication. The high cost can be justified by their relatively long life, which is made possible by the erosion resistance of the precious metal. Drain bushings also have been formed of precious metal, but their high cost cannot be justified since their intermittent use for only five to ten days at a time results in the precious metal being tied up during long periods of inactivity. It would be better to use drain bushings formed from nonprecious metal, provided they are able to adequately withstand erosion from the molten glass at least until the draining operation is over, and provided they can be adequately heated to the required temperatures.
The usual vertically arranged relatively thick lug-type terminal typically used with precious metal bushings has not been found to allow satisfactory electrical heating of a nonprecious metal drain bushing. The heat is not dispersed uniformly throughout the bushing, resulting in hot spots which are deleterious to the life of the bushing and which do not promote stable operating conditions. Other types of terminals, such as the conventional form of wing-ear terminals, have not been found to be satisfactory either. The desired heating patterns are still difficult to achieve and the terminal ears, when made thin enough to provide better heat transmission, display a tendency to bend when subjected to the pressures caused by the power clamp over a period of time. Attempts to use thinner terminal ears which have been reinforced against bending have not been entirely satisfactory since there is always the danger that the ear, or the metal strip to which the ear is connected, will not be able to resist the extremely high temperatures to which it is subjected and will burn through.